This invention relates in general to semiconductor devices and more particularly to semiconductor devices having packages including thermal dissipation structures and pressure venting capability.
Packaging for state of the art VLSI devices requires consideration for the dissipation of heat generated by the enclosed electronic component. Excessive heating during operation accelerates the rate of temperature sensitive failure mechanisms such as electromigration, bond fatigue, metal corrosion, and others. The most common method for providing thermal dissipation is to incorporate a heat sink into the package body that is in close proximity to the heat producing electrical component and molded into the package body. While the use of a heat sink provides an effective means of controlling the temperature of the semiconductor device during operation, problems exist with the implementation of a heat sink. The need for good thermal dissipation requires a large, thermally conductive body having a extensive surface area for heat transfer. The large volume of the heat sink can represent a limitation to the use of the semiconductor device in user systems with small dimensional requirements. Additionally, the large internal surface area of the heat sink, sealed to the package body by a conventional injection molding process, presents an extensive area upon which water and solvents, which have permeated through the package body and overlying adhesive layers, can condense and eventually vaporize causing catastrophic failure of the semiconductor device.
During the process of attaching a semiconductor device to a mounting substrate, such as a printed circuit board, a particular problem that must be considered is the potential for pressure buildup within the package body. Water and residual solvents trapped within the package body during the molding process or that have permeated through the package body, vaporize when the package body temperature rises, for example, when the package leads of the semiconductor device are soldered to contact lands on the printed circuit board. During the board mounting process the package body temperature can easily rise to temperatures in the range of 200.degree. to 250.degree. C. At these temperatures any water and many solvents within the package that have permeated through the package body and condensed on metallic or plastic surfaces, or within the package, will vaporize and can generate sufficient internal pressure to crack the package body. Package body cracking or rupture, as a result of internal gas pressure buildup is known in the art as "pop corn cracking" and can represent a major reliability problem.
Packages containing large metallic bodies, such as packages containing heat sinks or heat spreaders, present a large surface area upon which water and solvents may condense and thus are prone to large internal pressure buildup during board mounting. In a conventional assembly process, a heat sink is placed in the mold in close proximity to the integrated circuit device that is to be encapsulated. A mold compound is then injected into the mold enclosing the heat sink within the package body and sealing the metallic surface of the heat sink to the package body. Any water or residual solvents trapped within the package then can condense on the metal surface of the heat sink along the interface between the heat sink and the package body. The potential for gas pressure rupturing of the package body is increased by the large amount of metal surface area available for condensation to take place.
In an attempt to provide an effective heat sink which requires a large surface area for heat transfer to the ambient environment, heat sinks are often configured to have large portions that protrude well beyond the outer surface of the package body. The additional volume, away from the package body, that is occupied by the heat sink can present a problem for the user, who must often mount the semiconductor device on a printed circuit board that will be installed into a system having many tightly spaced printed circuit boards. The amount of head space between adjacent printed circuit boards varies greatly from one system to the next. Typically, the user wants to use the largest heat sink permitted by the application, however, semiconductor devices with heat sinks molded into the package body can limit the selection based upon the external dimensions of the heat sink rather than the particular performance characteristics of the enclosed electronic component. It would be advantageous if the user could select a heat sink profile that would be best suited to the amount of head space available in a particular system.
Thus, a continuing goal in the art of providing semiconductor devices having packages with thermal dissipation capability is a semiconductor device that will address these problems satisfactorily in an arrangement that can be readily manufactured at a low cost. It would be advantageous if a semiconductor device existed having good thermal dissipation capability with the ability to meet varying user space requirements and an ability to relieve internal gas pressure to avoid cracking or rupture of the package body.